This invention is generally directed to the cancellation of undesired echos on a two-way communications channel and is more specifically directed to an adaptive echo canceller which operates at passband frequencies. Passband refers to signals centered about a frequency other than zero hertz. Passband is normally contrasted with baseband in which signals are centered about or begin at zero hertz.
This invention is especially, but not exclusively, suited for use with modems which operate full duplex over a single transmission channel. In order to achieve reliable high speed data transmission it is necessary to cancel the echo of the local modem's transmitted signal from the received signal. Echo cancellers operating at baseband have been successfully used in modems. However, the digital implementation of a baseband echo canceller requires the use of complex arithmetic, i.e. calculations utilizing real and imaginary terms. Since the output of conventional modems is at passband, the baseband echo canceller must be located internally within the modem and operate on baseband transmit and received signals. Adaptive baseband echo cancellers are disclosed in U.S. Pat. Nos. 4,087,654; 4,355,214; and Re. 31,253.
Passband echo cancellers have been successfully utilized in voice communications paths, such as in the public telephone network, where the amount of echo cancellation required is not as great as in digital data applications. For example, U.S. Pat. Nos. 3,508,017 and 4,578,543 address echo cancellation for voice circuits. Such circuits have been found acceptable where cancellation of 30 decibels (db) to 40 db are acceptable. However, conventional passband echo techniques have not been able to achieve the same degree of echo cancellation as baseband cancellers which can achieve cancellation on the order of 60 db.
U.S. Pat. No. 4,464,545 addresses a passband echo canceller for data signals. A pair of filters which have output signals as a function of the single input signal stream is utilized.
It is an object of the present invention to provide an improved adaptive passband echo canceller which can provide levels of cancellation comparable to baseband echo cancellers.